DO NOT use as a general tag for sending email. This is about "sendmail", the general purpose email routing facility that supports mail transport via TCP/IP using SMTP
sendmail is a popular Unix-based email routing facility that makes use of and supports the transfer and delivery of email via the Simple Mail Transfer Protocol (SMTP).
sendmail exists both as free and open source software and as a commercial project, called Sendmail.
Tag usage
The tag sendmail can be used for all programming related problems. The tag can also be used for setting up a simple mail server. Please note that https://serverfault.com/ is another Stack Exchange website where you can ask especially configuration specific problems.